1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing a video signal, and more specifically to an apparatus for recording a video signal on a recording medium, such as a magnetic disk or the like, and for reproducing a video signal recorded on such a recording medium.
2. Background and Material Information
Apparatus are known for recording a video signal on a recording medium, such as magnetic disk, or the like, and for reproducing a video signal recorded on such a recording medium. For recording a video signal on a magnetic disk, for example, in such a video signal recording and reproducing apparatus, a luminance signal (having a synchronizing signal) and a color difference signal are frequency-modulated and then recorded on the magnetic disk.
One problem with the video signal recording and reproducing apparatus is that an actuator such as an electric motor for rotating the magnetic disk tends to suffer speed fluctuations due to mechanical disturbances or electric signal instability, for example, when a video signal is recorded on the magnetic disk and when a video signal is reproduced from the magnetic disk. Furthermore, a video signal recorded on a magnetic disk with one recording apparatus and reproduced using a different apparatus could also produce an unstable image due to differences between the recording apparatus and the reproducing apparatus. In the presence of such speed fluctuations of the actuator, the video signal recorded on the magnetic disk contains jitter introduced with high fidelity. Therefore, video images displayed on a display unit based on the reproduced video signal are subject to aberrations in size or position, and flickering.
As shown in FIG. 1, a video signal recording and reproducing apparatus 200 according to the present invention records a luminance signal with a synchronizing signal Y+S, color difference signals R-Y, B-Y and reference signals, through a Y signal recording circuit 50 on a magnetic disk 70. Specifically, an output signal from the Y signal recording circuit 50 is supplied to a magnetic head 60, which records the signal on a magnetic disk 70 that is being rotated by a spindle motor 80.
The recording and reproducing processes of the video signal recording and reproducing apparatus 200 will be described below. In FIG. 1, video signal recording and reproducing apparatus 200 is shown as recording and reproducing a video signal Y+S that is composed of the luminance signal Y and a horizontal synchronizing signal S. The synchronizing signal includes a horizontal synchronizing signal H-SYNC, vertical synchronizing signal V-SYNC and an equalizing pulse.
First, the recording process will be described. A video signal Y+S is supplied from a video camera (not shown) to a synchronizing signal separator 10 and also an adder 35. The synchronizing signal separator 10 separates the H-SYNC and V-SYNC signals from the video signal Y+S. The synchronizing signal separator 10 supplies the separated H-SYNC signal to a burst gate pulse generator 20. The burst gate pulse generator 20 generates a train of burst gate pulses each having a predetermined period which lags behind the corresponding pulses of the separated horizontal synchronizing signal by a certain period of time. The burst gate pulses are required to write a reference signal (described later on). The synchronizing signal separator 10 also supplies the separated horizontal synchronizing signal to a clock generator 40 and a start flag generator 42 (described later on). The clock generator 40 generates clock pluses as a reference signal (burst signal) for correcting jitter when the video signal is reproduced.
A burst gate 30 is supplied with the burst gate pulses outputted from the burst gate pulse generator 20 and the reference signal outputted from the clock generator 40. The reference signal may comprise, e.g., a sine wave having a frequency which may be 227.5 times the frequency of the horizontal synchronizing signal. The burst gate 30 passes the reference signal to the adder 35 while each of the burst gate pulses is in an ON state, (i.e., HIGH). Therefore, the reference signal is applied to the adder 35 only during a period of time in which each burst gate pulse is high. The adder 35 adds the reference signal to the video signal Y+S.
The video signal recording and reproducing apparatus shown in FIG. 1 further includes a start flag generator 42 which generates and outputs a start flag which lags behind a burst gate pulse based on the horizontal synchronizing signal from the synchronizing signal separator 10 and the clock signal from the clock signal generator 40. The start flag outputted from the start flag generator 42 is supplied to an adder 47, which adds the start flag to the video signal Y+S that incorporates the reference signal. The adder 47 outputs the video signal Y+S with the reference signal and the start flag to a Y-signal recording circuit 50. The Y-signal recording circuit 50 frequency-modulates and records the supplied video signal on the magnetic disk 70 through a magnetic head 60 while the magnetic disk 70 is being rotated by the spindle actuator 80.
FIG. 3 shows the timing relationship between a video signal with a reference signal and added start flags, burst gate pulses from the burst gate pulse generator 20, and start flags from the start flag generator 42. An effective horizontal period lies between a start flag and the subsequent horizontal synchronizing signal.
To reproduce a recorded video signal representing a still image from the recording medium, video information is first stored in a memory based on a reproduced video signal, and thereafter the stored video information is read from the memory and applied to a display unit. For writing the video information in the memory based on the reproduced video signal, a start flag indicative of a position to start writing the video signal, i.e., a point of time where an effective horizontal period starts, is added to the video signal, and the effective horizontal period is detected by detecting the added start flag.
The reproducing process will be described in detail below. The video signal Y+S recorded on the magnetic disk 70 is read by the magnetic head 60 and then frequency-demodulated by a Y-signal reproducing circuit 90. The frequency-demodulated video signal Y+S is supplied from the Y-signal reproducing circuit 90 to a synchronizing signal separator 100, a start flag detector 102, a clock signal reproducing PLL (phase-locked loop) 120, and an A/D (analog-to-digital) converter 130. The synchronizing signal separator 100 separates the H-SYNC signal from the video signal Y+S. The synchronizing signal separator 100 supplies the separated H-SYNC signal to a burst gate pulse generator 110. The burst gate pulse generator 110 generates a train of burst gate pulses each having a predetermined period which lags behind the corresponding pulses of the separated H-SYNC signal by a certain period of time. The burst gate pulses are required to extract the reference signal (burst signal) from the video signal Y+S. The burst gate pulse generated 110 supplies the burst gate pulses to the clock signal reproducing PLL 120. The clock signal reproducing PLL 120 extracts the reference signal from the video signal Y+S, and generates a clock signal in synchronism with the reference signal. The clock signal generated by the clock signal reproducing PLL 120 includes jitter which may have been introduced due to speed fluctuations of the actuator 80 when the video signal Y+S is recorded on and/or reproduced from the magnetic disk 70. Therefore, the clock signal is not of a constant frequency but has frequency fluctuations in synchronism with the video signal Y+S that is read from the magnetic disk 70. The clock signal generated by the clock signal reproducing PLL 120 is applied to the A/D converter 130 and a memory 140.
The start flag detector 102 detects a start flag from the reproduced video signal Y+S. When the start flag detector 102 detects a start flag, it applies an output signal to a memory write signal generator 104, which outputs a memory write signal from a predetermined period of time, i.e., an effective horizontal period, to permit video information to be written into a memory 140.
More specifically, the video signal Y+S supplied to the A/D converter 130 is converted into a digital video signal based on the supplied clock signal which contains jitter information. The digital video signal is then stored into the memory 140. Thus, the digital video signal is stored in the memory 140 based on the clock signal that is synchronous with the frequency fluctuations of the video signal (i.e., having the same jitter). Therefore, the resulting video signal stored in the memory 140 is free from jitter since the jitter in the video signal is canceled by the jitter in the clock signal. The jitter is thus canceled when the video signal is written to memory, using the clock signal which is generated by the PLL 120.
The video signal stored in the memory 140 is then read out using a clock signal having a constant frequency which is generated by a clock generator 160. The read video signal is converted into an analog video signal by a D/A (digital-to-analog) converter 150 also based on the clock signal from the clock generator 160. The analog video signal is thereafter supplied to an adder 175, which combines the supplied analog video signal with a synchronizing signal that is generated by a synchronizing signal generator 170 based on the clock signal from the clock generator 160. The video signal thus combined with the synchronizing signal is then outputted from the adder 175 to a display unit (not shown).